A silicon wafer for a solar cell is conventionally manufactured by thinly slicing unidirectionally solidified ingots of silicon. The quality and cost of a silicon wafer are dependent on the quality and cost of solidified ingots of silicon. Thus, improving the quality of a silicon wafer and reducing the cost thereof will require that high quality unidirectionally solidified ingots of silicon should be manufactured at a low cost, and as this method, the applicant of the present invention has put a silicon continuous casting method using electromagnetic induction heating to practical use.
The silicon continuous casting method using electromagnetic induction heating uses an electrically conductive bottomless crucible 3 which is placed inside an induction coil 2 and at least part in the axial direction of which is divided in the circumferential direction as shown in FIG. 15. In a production run, silicon raw materials supplied into the bottomless crucible 3 are melted by induction heating with high frequency power supplied to the induction coil via the bottomless crucible 3, and while silicon melt 19 is solidified, it is extracted down below the bottomless crucible 3 and supply of raw materials into the bottomless crucible 3 is continued. In this way, unidirectionally solidified ingots of silicon 12 (hereinafter simply referred to as “solidified ingots of silicon”) are manufactured continuously.
By dividing at least part in the axial direction of the bottomless crucible 3 in the circumferential direction, this method not only allows the silicon raw materials in the bottomless crucible 3 to be melted by electromagnetic induction heating but also allows the silicon melt 19 originated from the melting to generate a repulsive force against the bottomless crucible 3, thus reducing contact between the two, which makes it easier to extract the solidified ingots of silicon 12 and reduces pollution of the solidified ingots of silicon 12 by the bottomless crucible 3.
From the standpoint of the product quality, such a silicon continuous casting method is required to supply silicon raw materials of high quality with minimal impurities into the bottomless crucible. However, high quality silicon raw materials are expensive, and therefore silicon raw materials of low quality with relatively high percentage of impurities are required from the standpoint of the manufacturing cost. As the method of solving this contradiction, a method of refining silicon in a casting process by blowing a plasma gas onto the surface of the silicon melt in the bottomless crucible is disclosed in Japanese Patent Laid-Open No. 4-130009.
This method combines melting by electromagnetic induction heating and refining using a plasma gas, but a refining method using a plasma gas without using electromagnetic induction heating is also disclosed in Japanese Patent Laid-Open No. 11-49510, etc.
The plasma in the silicon continuous casting method combining melting by electromagnetic induction heating and refining using a plasma gas has not only the refining function but also the function as an effective heating source for melting silicon raw materials in the bottomless crucible. For continuous casting by electromagnetic induction heating, a secondary heating source is required to perform initial melting, etc. of silicon raw materials in the bottomless crucible. An electron beam, for example, is used as this secondary heating source, but heating by an electron beam requires decompression in a chamber, whereas plasma heating allows an operation at normal atmospheric pressure. Taking note of this advantage of plasma heating, the applicant of the present invention is proceeding with the development an electromagnetic induction casting method using plasma, transferred plasma arc in particular, as a secondary heating source as well.
On the other hand, in order to improve the performance of solidified ingots of silicon as a solar cell, it is effective to perform control in such a way as to minimize temperature gradient during the manufacture of solidified ingots of silicon in a temperature range from 1420° C., which is the melting point of silicon, to 1100° C. In connection therewith, the applicant of the present invention presented a “Method of manufacturing polycrystalline solidified ingots of silicon for a solar cell characterized by controlling temperature gradient to a range of 15 to 25° C./cm when silicon passes through temperature range of 1420° C. to 1200° C. in manufacturing polycrystalline solidified ingots of silicon to be supplied to solar cell through unidirectional solidification” in Japanese Patent Laid-Open No. 4-342496.
The reason why a reduction of temperature gradient in the temperature range from 1420° C. to 1100° C. is effective for improving the performance of a solar cell is that many crystalline defects occur when silicon passes through the temperature range from 1420° C. to 1100° C., which deteriorates the conversion efficiency of the solar cell and reducing the temperature gradient in this temperature range will reduce thermal stress generated inside crystals and prevent crystalline defects, etc.
Japanese Patent Laid-Open No. 4-342496 controls this temperature gradient and the temperature gradient here refers to temperature gradient in the axial direction of solidified ingots of silicon. According to the subsequent studies by the applicants of the present invention, what actually determines thermal stress is temperature gradient in the direction of the radius of solidified ingots of silicon and it has been discovered that it is necessary to reduce the temperature difference between the central area of an ingot and the surface of the ingot as close as to 0 in a high temperature area for the purpose of improving the performance.
In order to reduce temperature gradient in the direction of the radius of solidified ingots of silicon immediately after solidification according to the silicon continuous casting method using a bottomless crucible, it is necessary to reduce the amount of heat radiation from the side of ingots immediately after solidification. For this purpose, it is effective to keep warm the side of the solidified ingot immediately after solidification inside the bottomless crucible, and more specifically, it is effective to reduce the length from the lower end of the coil to the lower end of the crucible which constitutes the cooling section of the bottomless crucible. However, enhancing thermal insulation in such a way will increase the temperature of the surface of the ingot in the lower part of the bottomless crucible and when a certain temperature is exceeded, leakage of melt may occur due to breakage of the solidified shell. For this reason, if the amount of heat supplied from above the ingot is determined, a minimum amount of heat radiation available from the side is automatically determined within the range in which leakage of melt is prevented.
In case of the continuous casting method using electromagnetic induction heating, the silicon melt starts to solidify from the lower end level of the induction coil. The amount of heat necessary to melt the silicon raw materials to be supplied is only supplied by induction heating, and therefore convection of the silicon melt by an electromagnetic force is more conspicuous than when other heating methods are used, and as a result, the downward heat flow rate increases and a solid-liquid interface takes a concave shape deeply recessed downward. When the casting speed is further increased, the amount of induction heating increases and therefore convection becomes more conspicuous and the downward heat flow rate increases, which causes the concave shape of the solid-liquid interface to grow noticeably. As a result, the temperature in the central area does not reduce for a long time and the temperature gradient in the direction of the radius of the solidified ingot immediately after solidification increases.
In addition, when the concave shape of the solid-liquid interface grows noticeably, the solidified shell becomes thinner and it is more difficult to keep warm the side of the solidified ingot immediately after solidification and to increase the amount of heat radiation from the side, the length from the lower end of the induction coil to the lower end of the crucible which constitutes the cooling section of the bottomless crucible is increased. As a result, heat loss of the ingot which faces the surface of the crucible over a wide range immediately after solidification is promoted, causing considerable deterioration of quality.
In addition, in case of electromagnetic induction heating, an induction current flows near the surface of the silicon melt facing the inner surface of the crucible, and therefore most of Joule heat is generated near this surface. For this reason, the additional raw materials supplied into the silicon melt move close to the surface of the melt and start to melt there and the unmelted raw materials remain in the central area of the melt in the form of an island. Furthermore, because of electromagnetic force that acts on the melted silicon, the upper surface thereof upheaves and separates from the induction coil. This prevents an increase in the melting output from effectively contributing to an increase of the melting capacity. Thus, solubility of additional raw materials cannot be said to be sufficient.
On the other hand, as mentioned above, the applicant of the present invention is proceeding with the development of the electromagnetic induction casting method using plasma, transferred plasma arc in particular, concurrently as the secondary heating source. In the process of this research and development, it has been discovered that the concurrent use of transferred plasma arc is very effective in solving the above-described various problems accompanying electromagnetic induction heating.
That is, concurrent use of plasma heating for melting of raw materials in the middle of casting can reduce the load of electromagnetic induction heating and the reduction of load suppresses heat convection of melted silicon by electromagnetic force, suppresses the downward heat flow rate, flattens the solid-liquid interface and relaxes the concave shape. As a result, temperature gradient in the direction of the radius of the solidified ingots of silicon immediately after solidification is reduced. Furthermore, the solidified shell becomes thicker, which allows heat insulation of the side of the solidified ingot immediately after solidification to be enhanced and this heat insulation enhancement will also reduce temperature gradient in the direction of the radius.
Especially, in case of transferred plasma arc, it is easy to obtain large output necessary for casting silicon, and on top of it, an arc current flows through the solidified ingot of silicon which is the opposite electrode, and the Joule heat thereby generated is expected to have the effect of keeping warm the solidified ingots of silicon immediately after solidification from the inside thereof. Moreover, prevention of deterioration of solubility of the additional raw materials which presents a problem for electromagnetic induction heating can also be expected.
However, in case of conventional plasma heating used concurrently with continuous casting of silicon, heating is always performed on the central area of silicon melt in the bottomless crucible. To maximize the effects of plasma heating, it is effective to reduce electromagnetic force to the extent to which extraction of solidified ingots of silicon is not hampered and increase the load of plasma heating accordingly. In that case, it has been discovered that fixed heating of the central area of the silicon melt causes heat to be concentrated on the central area, making the concave shape of the solid-liquid interface grow noticeably and preventing sufficient improvement of the performance.
Furthermore, when continuous casting of silicon is performed by using transferred plasma heating concurrently with electromagnetic induction heating, it has also been discovered that the following problems occur in connection with the current-carrying path of the plasma current.
When silicon raw materials to be supplied into the bottomless crucible are heated and melted using transferred plasma arc, a plasma arc torch is inserted from above into the bottomless crucible to generate plasma arc between the torch and the silicon melt in the bottomless crucible. To generate this plasma arc, it is necessary to connect the torch to one electrode of a plasma power supply and electrically connect the other electrode to the solidified ingot of silicon extracted down below the bottomless crucible to supply power.
With regard to the power supply structure on the ingot side, Japanese Patent Laid-Open No. 11-49510 describes a case where a plasma electrode on the ingot side is attached to an extractor connected to the lower part of the solidified ingot of silicon. However, the solidified ingot of silicon extracted from the bottomless crucible itself becomes a resistance between the top end at which the solidified ingot of silicon contacts the silicon melt and the power supply section at which it contacts the plasma electrode, generating Joule heat caused by the plasma current. The specific resistance of silicon for a solar cell is on the order of 0.5 Ωcm to 2.0 Ωcm at normal temperature and this specific resistance increases as the temperature rises from normal temperature and reaches a maximum near 200° C. However, if the temperature further increases, the specific resistance decreases contrarily and reduces by triple-digits near the melting point (1410° C.) compared with normal temperature.
Attaching the plasma electrode on the ingot side to the extractor causes the solidified ingot of silicon to generate heat over the total length, but the total length of the ingot is not stable in case of continuous casting resulting in unstable heat generation. This causes seriously adverse influences on the quality of the ingot.
In the case where the total length of the solidified ingot of silicon increases as the casting proceeds, the current-carrying distance from the top end of the solidified ingot to the power supply section increases and electric resistance also increases, and therefore it is necessary to increase the output (voltage) of the plasma power supply considerably to give sufficient heat of melting to the silicon melt on the solidified ingot. This voltage increase deteriorates the thermal efficiency during casting and causes quite a big problem in manufacturing solidified ingots of silicon of high quality at low costs.
Attempting to reduce the chamber height and secure a long length of solidified ingot requires the power supply section to be located under the chamber. When power is supplied under the chamber, the solidified ingot of silicon is extracted from the chamber without decreasing its temperature and a drastic temperature variation may cause cracking on the surface. To continue continuous casting, it is necessary to cut the solidified ingot of silicon under the chamber every time a certain length of ingot is cast, but since the temperature at the cut section increases, there is a danger that the cutting blade will not be able to withstand the heat, making the cutting difficult.
It is an object of the present invention to provide a silicon continuous casting method capable of suppressing concave shaping of the solid-liquid interface, which presents a problem in silicon continuous casting using electromagnetic induction heating, also suppressing temperature gradient in the direction of the radius of solidified ingots of silicon immediately after solidification even in case of fast casting, improving the performance and thereby manufacturing high quality solidified ingots of silicon at lower costs.
It is another object of the present invention to provide a silicon continuous casting method capable of stably suppressing power loss and heat generation of solidified ingots when power is supplied to the ingots, which presents a problem when plasma heating using transferred plasma arc is used and thereby manufacturing high quality solidified ingots of silicon at lower costs.